1. Field of the Invention
This invention relates to an apparatus for lapping semiconductor wafers and a method of lapping thereof and more particularly to a semiconductor wafer lapping method of lapping sliced wafers.
2. Description of the Related Art
A sliced semiconductor wafer is chamfered, then the sliced surface of the semiconductor wafer is shaped by lapping or surface grinding.
The lapping is batch processing of a plurality of semiconductor wafers and is the best in processing efficiency. As shown in FIG. 4, in the lapping, a lapping carrier 4 having holding holes loaded with a plurality of semiconductor wafers is placed between an upper press platen 31 (not shown) and a lower press platen 3 which generally rotate in opposite directions, and is rotated by the rotation difference between a sun gear 51 and an internal gear 52 and is making sun-and-planet motion relative to the upper press platen 31 and the lower press platen 3 for lapping the semiconductor wafers 10. At the time, the semiconductor wafers 10 themselves also rotate and are flat ground.
For the rotation directions of the lapping carrier 4, the direction opposed to the rotation of the lower press platen 3 is forward rotation (arrow X) and its opposite direction is reverse rotation (arrow Y).
When the upper and lower press platens are mounted on a lapping apparatus, their surfaces are not necessarily flat and when lapping is repeated, the surface shapes furthermore change like a convex lower press platen 3a called "A" warpage as shown in FIG. 7(a) or a concave lower press platen 3a called "B" warpage as shown in FIG. 7(c). The upper press platen rotates following the shape of the lower press platen due to the tare weight of the upper press platen, thus the shape of the upper press platen depends on the shape of the lower press platen.
As a method of correcting the warpage of the press platen surface to a flat surface as shown in FIG. 7(b), the shape is measured, for example, once a day or a week, a correction carrier (not shown) is placed in place of the lapping carrier, and the lapping apparatus is rotated, whereby the press platen is shaped flat.
In this method, for the "A" warpage (convex), if the correction carrier is forward rotated with respect to the lower press platen 3 (arrow X :positive rotation), the hatched parts of the lower press platen are easily ground and the warpage is furthermore enlarged, as shown in FIG. 8(a). On the other hand, the correction carrier is reversely rotated with respect to the lower press platen 3 (arrow Y: negative rotation), whereby the portion corresponding to the projection of the lower press platen 3 is easily ground and the lower press platen 3 can be made almost flat, as shown in FIG. 8(b). In contrast, for the "B" warpage (concave), the correction carrier is forward rotated with respect to the lower press platen 3 (arrow X), whereby the lower press platen 3 can be made almost flat.
However, the shaping of the press platen using the correction carrier is poor in work efficiency, thus the measurement and shaping frequencies must be lessened. Meanwhile, the flatness of lapped semiconductor wafers is impaired gradually as the lapping is repeated.
Then, in the conventional lapping, a method of alternately inverting the lapping carrier rotation direction is proposed in Japanese Patent Laid-Open No. Hei 3-251363. This method can prevent the press platen from becoming deformed at the initial stage of the lapping process.
However, such semiconductor wafers ground in batch, which differ from each other slightly in thickens, come in contact with the upper and lower press platens in a projection area from the lapping carrier face and rotate due to friction; when lapping is repeated and the thickness becomes uniform, most semiconductor wafers do not rotate as they are sandwiched between the upper and lower press platens. Thus, as shown in FIG. 4, the same extensions 10a of the semiconductor wafers 10 are lapped while they extend off the lower press platen 3. Then, as shown in FIG. 11, only the portion abutting the marginal portion of the upper press platen 31 is much lapped and the extension 10a and an inner peripheral portion 10b positioned in the inner peripheral portions of the upper and lower press platens 3 and 31 are not much lapped, producing a recess 20a in the outer peripheral portion, resulting in a shape as shown in FIG. 10; the total thickness variation (TTV)TTV becomes a large value.